The semi-metals germanium and silicon have high transparency and high refractive indices over a wide range of infrared wavelengths and are stable to normal atmospheres. Their relative indices and dispersions make achromatic combinations possible; and designs are given for axially corrected doublets of relative apertures f:2 and f:1. The optical homogeneity of the materials is discussed: compositional variations are not considered an optical hazard, but there is evidence of structural imperfections in some specimens whose effect on optical properties remains to be evaluated.
© 1951 Optical Society of AmericaFull Article | PDF Article
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